feat(dart): build the web target's assets instead of scripting them

[nikzen]

A Flutter web application needs two compiled artefacts in web/ that no
repository should carry: the vodozemac WebAssembly module with its glue, and
LiveKit's frame-encryption worker. famedly/call produces them with a
prepare-web.sh that clones two repositories, cargo installs a code
generator and pins the worker to a branch name by hand. This replaces that.

• The bindings become a package. The native library and the WebAssembly
  module are the same crate compiled twice, so both now come from one pinned
  source and the Dart package can never talk to bindings from another revision.
  Everything vodozemac moves under nix/dart/vodozemac/ on the way.
• The worker is compiled from the package the project already resolved. Its
  location comes out of .dart_tool/package_config.json, so it follows
  pubspec.lock and needs no pin of its own.
• One script does both, and the devshell carries it. An asset only CI knows
  how to produce is one a developer cannot reproduce when the site misbehaves in
  the browser.

wasm-pack runs directly rather than through flutter_rust_bridge_codegen build-web, which is a Dart program that only shells out to it — that keeps a
Dart toolchain and a pub cache out of the build. The price is that the flags
live here, and flutter_rust_bridge has changed them between versions, so the
build fails with an explanation if the crate stops pinning the version those
flags were read off.

Stacked on #56, whose build.nix this extends.

Test plan

• nix flake check --all-systems
• nix build .#famedly-vodozemac-web: the module carries the threading
  instructions the browser needs, and the two files match byte for byte what
  prepare-web.sh produces today
• ran the generated script in famedly/call: both assets land where they are looked for, the
  worktree stays clean, under two seconds
• the generated workflow runs it before flutter build web
• a full web build in CI

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Pub get pulls example deps

Medium Severity

Issue: When package_config.json is missing, the LiveKit asset step runs flutter pub get without --no-example, so CI also resolves a project's example/ pubspec. That can fail or drift the lockfile on apps that ship an example, and it contradicts the checks workflow which already avoids this.

Fix: Use flutter pub get --no-example.

 

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